Solid state lenses and lens components

ABSTRACT

Solid state lenses, lens blanks, and lens components formed of compressible mixed powders of two or more optical substances where the mixing ratio has been deliberately varied from place to place within the volume of the optical component to produce optically useful variations in the local refractive. The compressible mixed powders are ground as fine powders having mechanical properties that make them capable of being formed into cohesive monolithic masses that are low in scattering. The fine powders may be admixtures of host matrix materials and others which, when combined, provide preferred optical properties such as index and dispersion. Parts possessing transmission from within the range from the ultraviolet to the infrared are possible. The materials are suited to low temperature formation of aspheric lenses transmissive in the near and far IR.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/410,776 filed on Apr. 25, 2006 in the name of William T. Plummer andentitled SOLID STATE METHOD AND APPARATUS FOR MAKING LENSES AND LENSCOMPONENTS, the entire contents of which are incorporated herein byreference.

FIELD OF THE INVENTION

This invention in general relates to solid state methods and apparatusfor manufacturing lenses, lens blanks, and lens components and moreparticularly, to low temperature fabrication of aspheric lenses designedfor the transmission of ultraviolet, visible and/or infrared light.

BACKGROUND AND INVENTION

Traditional lens grinding and polishing technologies can most easilymake optical surfaces that are portions of spheres or simple flats.(Strong, John, Procedures in Experimental Physics, Chapter 11,Prentice-Hall, New York (1938).) When a crude spherical surface isrubbed repeatedly and randomly against a matching crude sphericalsurface, with the interface filled with a slurry of small abrasiveparticles, irregularities are worn off, and both surfaces become moreaccurately spherical. The natural ease by which spherical opticalsurfaces can be made was also expressed by F. Twyman, Prism and LensMaking, Hilger and Watts, London (1952), and by D. F. Horne, OpticalProduction Technology, Crane Russak, New York (1972).

Many examples can be listed in which the performance of an opticalsystem is improved through the use of one or more non-sphericalrefractive surfaces. “An aspheric surface can be a powerful design toolfor the reduction of residuals or the elimination of primary aberrations(especially distortion, astigmatism, and spherical) which will yield tono other design techniques.” (Smith, Warren J., Modern OpticalEngineering, page 351, McGraw-Hill, New York (1966).) But as Smith putsit, “Aspherics, cylinders, and toroids do not share the universality ofthe spherical surface, and their manufacture is difficult. While asphere is readily generated by a random grinding and polishing (becauseany line through the center is an axis), optical aspherics have only [atmost] one axis of symmetry. Thus, the simple principle of randomscrubbing which generates a sphere must be replaced by other means.

An ordinary spherical optical surface is a true sphere to within a fewmillionths of an inch. For aspherics this precision can only be obtainedby a combination of exacting measurement and skilled hand correction.”(Op. cit., page 413.) “In almost all cases, the designer is restrictedto the use of spherical refracting or reflecting surfaces, regarding theplane as a sphere of infinite radius. The standard lens manufacturingprocesses generate a spherical surface with great accuracy, but attemptsto broaden the designer's freedom by permitting the use of nonspherical,or “aspheric”, surfaces lead to extremely difficult manufacturingproblems; consequently such surfaces are used only when no othersolution can be found.” (Kingslake, R., Lens Design Fundamentals,Academic Press, New York (1978).)

The extra work required to generate and polish an accurate asphericsurface may be worthwhile if that surface can be used as part of a mold,to manufacture large numbers of lenses with the desired non-sphericalshape. Aspheric lenses, Fresnel lenses, and diffractive optical elementsare commonly manufactured for special system requirements by injectionor compression molding of thermoplastic optical polymers such as PMMA,polystyrene, or polycarbonate, or by casting a transparent epoxy orthermoset material in such an aspheric mold. But, unless the opticalcomponents are quite thin, such polymers are severely limited in theirinfrared transmission, typically to less than 1.7 microns wavelength, bymolecular resonance bands, and may be limited to 0.300 micronswavelength or longer in the ultraviolet.

Examples of polymer spectral transmission measurements can be found inthe USPL Handbook of Plastic Optics (United States Precision Lens,Cincinnati, second edition, p. 20 (1983).) (Also see:http://www.gsoptics.com/custom_optics/charts.html (illustration copiedhere) and http://www.ircon.com/pdf/wtn100.pdf for transmission data forpopular plastics, and FIG. 2)

There are also moldable glasses that can be used for manufacturingaspheric lenses, such as those available from LightPath Technologies,Inc. (Geltech) of Orlando, Fla., but these materials are also severelylimited in their infrared and ultraviolet spectral transmission range.

In addition, there are many exotic crystals, alloys, and other materialsavailable that can be ground and polished for use as lenses, many ofthem transparent across much larger parts of the infrared andultraviolet spectrum, but these materials are not considered to besuitable for volume manufacture by molding in any of the usual ways, andsome of them are quite expensive to obtain as raw material. (Tosi, J.L., Optical Materials: Making the Right Choice in the IR, in ThePhotonics Handbook, p. 391 ff., Laurin Publishing, Pittsfield, Mass.(2003).) Aspheric optical components can be made from some of thesecrystalline and amorphous alloy materials by Computer-Numerical-Controldiamond surface cutting or grinding, or by skilled use of the oldermanual grinding and polishing procedures mentioned by Strong (op. cit.)and by Smith (op. cit.), but all of these processes may be too slow andexpensive for economical high-volume production.

Tosi (op. cit.) characterized the wide range of physical properties ofthe useful infrared optical materials, about half of which arechemically alkali or other metal halides. Most of them can be ground andpolished optically as if they were glass, but there are significantdifferences. Some are quite brittle, some fracture easily when theirtemperature is changed, some corrode the materials in contact with them,some melt at very high temperatures, and some decompose before meltingif they are heated. They cannot be molded in the manner of thermoplasticresins and cannot easily be cast in desired shapes.

The only infrared optical materials commonly thought to be “moldable” tonon-spherical component shapes are the two comparatively heavy andexpensive proprietary chalcogenide glasses, Ge₂₂As₂₀Se₆₈ andGe₂₀Sb₁₅Se₆₅, available commercially from Umicore, a European company.These materials are chemically similar to AMTIR-1 in Tosi's list.

Consequently, it is a primary object of this invention to provide solidstate methods, materials, and apparatus by which lenses, lens blanks,and lens components useful in transmitting ultraviolet, visible, and/orinfrared light.

It is another object of this invention to provide methods, materials,and apparatus by which lenses, lens blanks, and lens components can beformed at temperatures within the range from approximately roomtemperature to less than the melt temperatures of the materials.

It is yet another object of this invention to provide aspheric lensesand lens components fabricated with solid state, low temperatureprocesses from specially prepared powders.

It is another object of this invention to provide protective barriersand methods for protecting lenses formed in the solid state againstmoisture and other environmental effects.

It is another object of this invention to provide materials that can beground into powders suitable for forming lenses, lens blanks, and lenscomponents by compressing them into cohesive monolithic solids attemperatures less than their melt temperatures.

Other objects of the invention will, in part, appear hereinafter and, inpart, be obvious when the following detailed description is read inconnection with the drawings.

SUMMARY OF THE INVENTION

The present invention is directed to methods, materials, and apparatusfor manufacturing lenses in the solid state, preferably at lowtemperatures. Essentially, the method involves a solid state near roomtemperature process in which deformable powder particles are squeezedtogether so well under compression that they nearly fill the volumeavailable to them, cling together in the form of a solid, and pass areasonable amount of radiation without scattering it. The powders areground from suitable optical materials that provide transmission ofultraviolet, visible and/or infrared light and possess the capability ofbeing formed into a solid cohesive mass having low absorption andscattering over the desired operating wavelength region of the finallens or component.

The inventive method includes grinding optical materials into powderform and placing the materials into a compression die shaped accordingto a predetermined lens, blank, or component profile. After closing thedie, sufficient mechanical pressure is applied to form the lens whilevacuum pressure is preferably applied to remove residual air from thecompressed powder. The events that occur in the process of compressionare believed to be: (1) transitional repacking. (2) deformation powdercomponents at points of contact, (3) fragmentation and/or deformation,(4) bonding, (5) deformation of the solid body, (6) decompression, and(7) ejection.

Admixtures of a suitable host matrix material and other materials havingpreferred optical properties may be used to achieve, for example,desired index of refraction and dispersion. The powder nanoparticles arepreferably less than about one-tenth of the smallest wavelength withinthe operating wavelength range of the final lens. For use in theinfrared, the particles can be estimated to be small enough when they donot have any facets, in the case of crystalline powders, that specularlyreflect glints of visible light.

Host matrix materials and IR materials may be selected from the alkalior other metal halides, and potassium bromide is particularly suitable.

The method may be used to fabricate blanks of suitable hardness fromwhich lenses can be fabricated by conventional machining techniques.

Protective barriers such as a thin films, e.g., Saran® may be appliedafterwards to inhibit moisture and other environmental effects.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure, operation, and methodology of the invention, togetherwith other objects and advantages thereof, may best be understood byreading the following detailed description in connection with thedrawings in which each part has an assigned numeral or label thatidentifies it wherever it appears in the various drawings and wherein:

FIG. 1 is a diagrammatic elevational view of commercial die set that isreadily modifiable to practice the invention;

FIG. 2 is a graph showing the transmission characteristics of variouspolymers;

FIG. 3 is a graph showing the transmission characteristics of PVDC(“Classic”) Saran film;

FIG. 4 is a graph showing the transmission characteristics of PE(“Premium”) Saran film;

FIG. 5 is a drawing showing the use of a removable liner between the dieset and a lens; and

FIGS. 6 a and 6 b are drawings showing the use of a die body itself as amount for a compound lens system.

DETAILED DESCRIPTION

Central to this invention is the realization that lenses, eitherspherical or aspheric in shape, can be formed usefully and economicallyin the solid state, preferably at or near room temperature, from manyuseful materials by a novel use of a well-known technique, a method thatis commonly used by infrared spectroscopists to embed powdered chemicalsamples in a flat solid matrix for spectroscopic examination, but amethod that is apparently not known for use in making lenses, lensblanks or lens components.

In this conventional technique, a metal die set is assembled to hold adried powdered matrix host material, such as potassium bromide or othermetal halide material, and constrain it to prevent lateral spreading.The matrix host material has added to it a small percentage of materialto be spectroscopically analyzed. Most of the air is removed from thepowder with a simple vacuum pump. The flat-faced metal die is firmlycompressed to consolidate all of the powder into a solid mass, whichbecomes reasonably clear like a small window.

FIG. 1 from Sigma-Aldrich Corporation, St. Louis, Mo., shows the simpleelements of one commercial die set used for forming transparent samplingpellets of potassium bromide and other alkali halides for infraredspectroscopy. I propose using a die set similar to this, but modifyingit so that the plunger and base facing surfaces shown as 10 and 12,respectively, carry spherical or aspherical shapes to aid in theformation of lens or lens components.

These are the specific steps typically suggested for forming a potassiumbromide optical sampling pellet in this classical way with thecommercially available equipment, and these can be used in modified formto carry out my invention as well. They are to:

-   1. Select a material that will serve as a host matrix or serve as    the final material composition (e.g. potassium bromide) of the    desired optical component or lens and select with it as needed    another material to alter the optical properties of the host    material to satisfy the design requirements of the final product;-   2. Individually grind the material(s) of step 1 with an agate or    alumina mortar and pestle until there is no (visible) evidence of    crystals in the powder. Preferably the powdery nanoparticles will be    less than about one-tenth the wavelength (λ/10) of the smallest    wavelength within the operating wavelength range of the product. As    will be appreciated, powders of fine particles may also be formed by    mashing, abrading, attriting, subliming, melting and spraying, and    dissolving and spraying.-   3. Dry the material(s) in an oven at 110° C. for 12 hours and store    them in a desiccator until they are needed.-   4. Warm the compression plunger die and anvil slightly to be sure    that they are dry.-   5. Gently and thoroughly mix weighed amounts of the material(s) and    put the proper quantity into the open die set to form a pellet of    the proper thickness.-   6. Put the compression die set together with a small amount of    mechanical pressure and pull a vacuum on it for one to two minutes    to remove any residual air.-   7. Continue pumping the vacuum while applying 40,000 to 60,000    pounds/square inch of pressure for two minutes.-   8. Release the vacuum, then release the mechanical pressure.-   9. Take apart the die set.-   10. Push the finished compacted object out of the die set body.

For conventional spectrometric sampling, the material to be studied isthoroughly but gently mixed with the potassium bromide after step (3),with a relative concentration of 0.1 to 2%. Although potassium bromideis the most popular material used in this way, potassium chloride,cesium iodide, and high-density polyethylene, are also suggested forstudent use in forming sample pellets by the Keck InterdisciplinarySurface Science Center at Northwestern University.

Both Buck Scientific Corporation of East Norwalk, Conn., andInternational Crystal Laboratories of Garfield, N.J., offer a commercialhand-held Quick Press, accessories, and instructions for making pelletsup to 7 mm diameter. The Sigma-Aldrich Corporation of St. Louis, Mo.,offers a commercial “Aldrich KBr Die” suitable for making pellets up to13 mm diameter, and suggests that pressures of 25,000 to 40,000 poundsper square inch will be suitable for making clear pellets of potassiumbromide, thallous bromide, or cesium iodide. Commercial apparatus forusing this technology has been available from these and other suppliersfor more than forty years. This process of forming a solid disk bymechanical compression of a mixture of a powdered sample with powderedpotassium bromide has been accepted as a standard procedure foranalytical infrared spectroscopy (Reagent Chemicals: American ChemicalSociety Specifications, Official from Jan. 1, 2000, Ninth Edition,American Chemical Society and Oxford University Press, New York (2000)p. 78).

(See, for example:

-   http://www.sigmaaldrich.com/img/assets/3762/al_techbull_al191.pdf,-   http://www.nuance.northwestern.edu/KeckII/ftir4.asp,-   http://www.chemistry.nmsu.edu/Instrumentation/KBr_New2.html,-   http://www.thomasregister.com/olc/10493500/irsolid2.htm,-   http://www.internationalcrystal.net/icl70.htm,-   http://www.columbia.edu/ccnmtl/draft/dbeeb/chem-udl/solid_sampling.html,-   http://www.internationalcrystal.net/ti_sec6.htm and its following    pages.)

While certain host materials have been identified for use inconventional processes, others may serve for the purposes of the presentinvention. What is required is a host material that has the capabilityof being formed into a solid cohesive mass having low absorption andscattering over the desired operating wavelength region of the finallens or component. The host material will also have sufficient bindingproperties to enable it to have other optical materials added to it insufficient concentrations to provide desired design optical propertieswhile at the same time permitting the final product to remainessentially solid for its intended use. That is, the final compressedproduct should be capable of being self-supporting in use and thereforebe of monolithic form. Materials of needed optical properties that arealso high in cohesive surface energy density would be suitablecandidates since small particles of them tend to cohere with one anotherwhen brought into proximity under pressure.

When the conventionally flat surfaces of the compression die (theplunger and anvil) are replaced with optical tools fabricated andpolished with any desired optical shape, a lens can be satisfactorilyand advantageously formed in much the same way as is commonly used toform the flat solid sampling pellet matrix. Those optical forming toolscan be ground and polished to any chosen form and optical finish on astainless steel blank, or can be diamond-cut in a suitable nickel alloyplated onto a steel substrate. Both methods are familiar from common usein making both spherical and aspheric injection molding tools forpolymers, and have been available commercially for more than 30 years.(Plummer, W. T., “Unusual Optics of the Polaroid SX-70 Land Camera”Appl. Optics 21(2), 196-202 (1982), and Plummer, W. T., J. J. Mader, J.W. Roblee, and J. Van Tassell, “Precision Engineering at Polaroid” Proc.of the Pre-Conf. Day, pp. 24-29, Precision Engineering inIndustry—International State of the Art, Eighth Int. Precision Eng.Seminar, Universite de Technologie de Compiègne, France; M. Bonis, etal., Ed. (May 15, 1995)) Machines capable of generating spherical andaspheric lens mold shapes with the necessary mathematical complexity andaccuracy on a hard and tough surface are available from the Moore ToolCompany of Bridgeport, Conn., and from Precitech, Inc., of Keene, N.H.,and for use at the longer infrared wavelengths, a mechanically ground orlathe-turned optical finish may be adequate for present purposes withoutfurther polishing. Of course, as in a common polymer molding practice,the master shape can be generated first in a soft material, such as aplastic or an easily machined metal, then converted into a hard moldcomponent by nickel electroforming replication.

One specific “lens component” that can usefully be manufactured by theinvention is an array of many small lenses, such as spherical oraspheric lenses arranged in a square or hexagonal array, or an array ofmany cylindrical lenses lying parallel to each other on a surface. Anickel tool will work well for this purpose and may be made byelectroforming from a master of the correct shape formed in anyconvenient material. One example is the ruled patterns of cylindricallenses depicted in FIGS. 3 and 4 of U.S. Pat. No. 3,848,980 by Plummerand incorporated herein in its entirety by reference. Diffractivestructures such as those depicted in U.S. Pat. No. 5,260,828 by Londono,et al. that may be used for athermalization, achromatization, or beamsplitting can also be usefully manufactured in this way. The '828 patentis also incorporated herein in its entirety by reference. Anotherdiffractive structure that can be manufactured in this way might be adiffraction grating, or might be a computer-generated surface-relieftransmission hologram formed by use of a mechanical, microlithographic,or laser-scanned photo polymerization fabrication process, preferablyfollowed by an electroforming step to make a durable molding tool. Sucha hologram may be used for projecting or displaying a desired optical orinfrared image. The lens that can be manufactured with this powdercompacting technology can also be a Fresnel lens, or indeed, almost anyoptical component that can be molded of thermoplastic polymers usingtraditional hot methods. The “lens component” can further be an axicon,or other refractive or diffractive component as might be designed forproducing a Bessel (non-spreading) beam of radiation, or can be a prism,or a joined cluster of prisms.

The necessary forming pressure of 25,000 to 60,000 psi for practicingthis new lens-fabrication art can be achieved easily for small lenseswith a tabletop press, such as a 3-ton arbor press, or with a largermetal-forming press, or even with the fast-acting mold containment clampof a conventional injection-molding machine. To provide the necessarypressure for consolidating powder to make a larger-diameter spherical oraspheric lens, the required compression force will scale approximatelyas the projected area of the compression die, so a clamping force of 300tons will be required to form a lens of 3″ to 4″ diameter. Suitablepresses with a force capacity of 2000 tons or more are readily availablefrom sources such as the Beckwood Press Co. of St. Louis, Mo. A littleless force or time may be needed if the powder is heated moderatelyabove room temperature, but melting is not necessary, and some of thesematerials may decompose at high temperature. Because the powder will notflow laterally as easily as a liquid, more force will be needed if thereis significant thickness variation across the area of the lens.

The pressure and time for the process must be sufficient to collapse thespaces between the powder grains to an insignificant size for opticalscattering, preferably to a dimension no more than 1% to 10% of theintended wavelength, and thereby reduce the compressed mass to ahomogeneous and essentially clear optical lens. The optical formingtools may be over-plated as necessary with any tough and sufficientlyinert material to avoid corrosion by contact with the metal halide, orthe tools themselves may be made of more corrosion-resistant materialsby grinding and polishing glass, fused silica, or a hard ceramic insteadof the stainless steel that is more commonly used for polymer molding.Even an injection-molded polymer shape may be usable in the cylinder asa forming tool if it is placed between the plunger or the anvil and thepowder; the polymer would be more subject to mechanical wear than metalor ceramic, but would be inexpensive enough to discard after one use.

One way to maintain a nearly uniform thickness across the area of thecompacted powder is to insert into the press a pre-made transparentlens, with spherical surfaces finished in a conventional way, betweenthe powder and either the plunger or the anvil, such that the insertedlens provides most of the thickness variation. The compression surfacethat holds the back of the pre-made lens is fitted to it to distributethe high pressure evenly and avoid breakage. After the compression hassolidified the powder, the pre-made lens and the consolidated powder arethen treated as a single object when they are removed from the cylinderand mounted in an optical system.

Potassium bromide itself, the substance most commonly used as a matrixfor spectroscopy, is usable itself as a finished optical component withtransmission from about 0.22 micron wavelength in the ultraviolet tonearly 30 microns in the infrared. Other metal halide materials such ascesium iodide can be selected for use from 0.2 microns through 50microns. (The Photonics Handbook, Laurin Publishing, Pittsfield, Mass.(2003) p. 20)

Additionally, some metal halides such as silver chloride can be formedfirst as a flexible and ductile sheet, handled much like sheet lead,that can subsequently be formed into a lens, or other optical element ofalmost any desired surface shape, simply by compression-forming itbetween generated and polished optical molding tools, much as a coin isstruck between figured dies. If a sufficiently smooth sheet of metalhalide is used as the raw material, the lens can be formed quickly andeasily at or near room temperature by pressure alone. Depending on thematerials, temperatures ranging from room to less than the material melttemperatures can be used to practice the invention. Essentially anytemperature at which the materials are in their solid state is possible.For convenience and ease of use, it is preferred to practice theinvention within the temperature range from 40° F. to 520° F., and mostpreferably within +/−20° F. of room temperature.

It is further noted that this same lens fabrication art can be used toachieve useful new options in the optical materials from which a lenscan be made. Because the metal halides have excellent matrix-formingproperties when compressed, they can be used to make a variety of newmaterials that are mixtures of one or more metal halides with one ormore other substances having different optical properties. If such othersubstances are ground or otherwise pulverized to small particles,ideally having diameters of 1% to 10% of the wavelength of the opticalor infrared radiation to be passed, then Rayleigh and Mie scattering canbe made small. (Van de Hulst, H. C., Light Scattering by SmallParticles, John Wiley & Sons, New York (1957).) For particles muchsmaller than the wavelength of light, the amount of scattering will bereduced with the sixth power of the particle size, so undesired scattercan be effectively controlled in this way. Acceptable particle diameterscan still measure 100 to 1000 or more atomic spacings across, and thuseach particle may contain between a million and a billion atoms. But,the particles will still be so small and so numerous that a large numberof each kind of particle will be found within any localcubic-wavelength-sized volume element of the composite material.

Such a hybrid material will not be a true optical glass, in which thecomponents are mutually melted, dissolved, and intermixed to themolecular size level, but it will in most optical respects perform as ifit were a true glasslike optical material. In particular, its collectiverefractive index represents an “average” that may be calculated fromthose of its constituents by use of the familiar Lorentz-Lorenz formula.(See Jenkins, F. A., and H. E. White, Fundamentals of Optics, SecondEdition, page 251, McGraw-Hill, New York (1950).) The Lorentz-Lorenzformula is equivalent to the Clausius-Mossotti equation encountered inthe microscopic theory of dielectrics; see Reitz, J. R., and F. J.Milford, Foundations of Electromagnetic Theory, Addison-Wesley, Reading,Mass. (1960).) The combined refractive index n of the inventive hybridcompacted material can be represented fairly well by noting that theLorentz-Lorenz ratio P=(n²−1)/(n²+2) for the combination is the average,weighted by volumetric proportion Vi/V, of all the Pi values separatelycalculated for all the individual constituents. That is, P=ΣVi Pi, whereΣVi/V=1, where we perform the sum over the two or more substances beingcombined. This quantity P, and from it the calculated combinedrefractive index n, can be calculated for any useful wavelength in ourextended spectral range. Unlike a true glass, in which the constituentsmust exhibit a high degree of chemical compatibility for a stableproduct, these “pseudo glasses” have only about one ten-thousandth toone millionth the interacting surface area between one constituent andanother, and can therefore exhibit fewer problems with incompatibility.The “other substances” can therefore be almost any material withattractive optical properties, including amorphous materials such asglasses (e.g. chalcogenide), crystals, ceramics, metals, orsemiconductors.

Suppose, for an example of this calculation, that an optical designrequires use of a transparent material with a refractive index of 1.90for a lens working at the wavelength of 4.0 microns in the infrared.Compiled lists of the common infrared optical materials include nocandidates between Cesium Iodide at an index of 1.75 and Silver Chloridewith an index of 2.00 (Tosi, op. cit.). But two readily availablematerials are Potassium Bromide, with an index of 1.54 and aLorentz-Lorenz ratio of 0.3138, and Germanium, with an index of 4.00 anda Lorentz-Lorenz ratio of 0.8333. The Lorentz-Lorenz ratio needed forthe composite refractive index of 1.90 is 0.4652. Using the well-knownmixing formula stated above, the volume can be calculated as aproportion f of Potassium Bromide and the volume proportion (1−f) ofGermanium, which are related by the equation:P=ΣViPi=0.3138f+0.8333(1−f)=0.4652

By solving for f, we calculate that a finely divided uniform mixture off=70.86% by volume of Potassium Bromide, together with (1−f)=29.14% byvolume of Germanium, can be compacted to provide the desired refractiveindex of 1.90. Because we want the compacted material to cohere as asolid object, it is favorable that Potassium Bromide, an excellentmatrix-forming material, represents a majority (70.86%) of the finalvolume. For convenience in measuring the quantities to be mixed, thesevolume proportions can be converted easily to weight proportions by useof the respective densities of the two materials. Because the twocomponent refractive indices in this example are greatly different fromeach other, it will be especially important to keep the particle sizessmall to reduce optical scattering.

The same mixing calculation can be applied to combinations of three ormore materials that may be needed at times to obtain desired refractiveindices at two or more different wavelengths. In using such a mixture ofoptical materials we might follow the same classical specific stepstabulated above for potassium bromide, but with the first two stepsbeing carried out separately for each of the two or more constituents ofthe mixture, and with steps (3) and (5) understood to apply to theuniform mixture of the finely divided constituents.

For use with visible light there are a variety of useful glass materialswith a refractive index that is deliberately varied with position withina lens blank. Three basic types are used, with gradients that arespherical, radial, or axial, with index gradients that may range from 1%to 25% or 30% of the base index of refraction. (Shannon, R. R., The Artand Science of Optical Design, Cambridge (1997), pp. 595-6) Thesematerials offer additional capabilities to the lens designer, andsoftware provided by Optical Research Associates, Pasadena, Calif., byZemax Development Corporation, Bellevue, Wash., and by other firmsenables designers to make good use of this feature. In some cases gradedindex optical materials may replace aspheric surfaces in lenses.(Kingslake, R., Optical System Design, Academic Press (1983), p. 3.)Graded index optical products are available commercially from theGradient Lens Corp. of Rochester, N.Y.

In the infrared, a graded index material formed by vapor deposition ofvarying proportions of Germanium oxide and Silicon oxide is commonlyused to form optical fibers, but few materials have been available foruse in infrared lenses. This inventive new means of lens or lens blankfabrication by low-temperature compression of finely divided solidmaterials now offers attractive new options for making graded indexmaterials. Three of the most popular materials for forming a solidmatrix by compression are potassium bromide, potassium chloride, andcesium iodide, with respective refractive indices of about 1.53, 1.45,and 1.74 in the useful wavelength region from 8 to 13 microns. Thesematerials may be combined with each other in any proportions withoutcompromising their mechanical strength after compression, but othermaterials could be used. For any desired index within this large range,the Lorentz-Lorenz formula will provide the mixing ratio needed toachieve it with two or more of these ingredients.

For an example, suppose that a lens (or a lens blank for later opticalfinishing) is needed with an axial refractive index gradient rangingfrom 1.48 at one surface to 1.50 at the other. Such an object can bemade by pre-mixing batches of these powdered materials with refractiveindices at any number of steps from 1.48 through 1.50, using enoughsteps so that the index change at each one is small enough to beacceptable. The cylinder is then filled with these mixed powders onelayer at a time, each layer being gently dusted through a screen todistribute it evenly across the area of the cylinder, and each layerweighed or otherwise measured to control the quantity of powder in it.The compression plunger is inserted into the cylinder, air is removed,and the powder is compressed as before to form a solid object. Becausethere is independent control of the quantity of each mixture added, therefractive index gradient can have any required mathematical form. Ifnecessary an apparatus can easily be built to dispense the mixed powdersin an essentially continuous manner, rather than in a series of smallsteps, to make the index gradient as smooth as may be desired. Sphericaland radial gradient index distributions are more complicated toconstruct, but can be produced in much the same way by depositingmeasured quantities of the successive mixed powders into the appropriateplaces within the compression cylinder.

Most metal halide materials and some of the other materials useful forinfrared optical components are hygroscopic to some degree, butaccording to known art they can be protected from the atmosphere by athin evaporated coating of magnesium fluoride or a suitable polymer.Magnesium fluoride was historically applied by John Strong to protectthe surfaces of alkali halide prisms (an expired US patent). Strong alsosuggested the use of lacquer to protect faces of such a prism. (Op.cit., page 88, FIG. 40.)

A protective polymer coating can be applied by evaporative depositioncoating, or by reactive vapor coating, as exemplified by theparaxylylenes. Paralene or Parylene can be applied conformally to athickness of just a few microns on all sides of an optical componentthrough a dimer-monomer-polymer process, offered commercially byParylene Coating Services, Inc., of Katy, Tex., and illustrated athttp://www.paryleneinc.com/process.html, and in such a thin layer cancombine moisture protection with adequate spectral transmission across abroad optical and infrared spectrum.

A protective moisture barrier can alternatively be provided bypolyvinylidene chloride (PVDC, or Saran “Classic”) resins and films.Saran “Classic” can be applied by dipping the optical component orspin-coating it from a solvent, such as a 60%/40% mixture oftetrahydrofuran (THF) and toluene (TOL), using a preparation identifiedas IXAN PNE 613 supplied by SolVin S.A. of Brussels, Belgium. Or theoptical component can be protected by placement between two layers ofthin film PVDC material in a mechanical cell designed to permanentlyhold the two layers tightly together around the entire perimeter of thecomponent.

The protective moisture barrier can be configured as a heat-sealed thinfilm protective envelope, perhaps of polyethylene (PE). An enclosuremuch like a commercial vacuum-formed “blister” package may work well.Across the useful 8 to 14 micron infrared range, even a 150-micron thickfilm of polyethylene can transmit more than 60%, but a much thinnerlayer will protect against moisture. A transmission spectrum of“Classic” Saran (PVDC) showing its large practical wavelength range inthe infrared is given in:http://www.shimadzu.com.br/analitica/aplicacoes/FTIR/A323.pdf, and isreproduced here as FIG. 3 from Shimadzu Corporation, Tokyo, illustratingthe useful infrared spectral transmission range of “Classic” Saran(PVDC). The reciprocal scale on the x-axis extends from 2.5 microns(2500 nanometers) wavelength at the left to 20 microns (20,000nanometers) wavelength at the right. This material is also clear invisible light and for a useful range into the ultraviolet as well. FIG.4 displays the measured transmission spectrum of “Premium” Saran (PE),in the form of a film about 12 microns thick, which offers generallybetter spectral transmission than PVDC, and can also be used as aprotective envelope, but is less suitable than PVDC for dip coating orspin coating. The approximately sinusoidal transmission variation inFIG. 4 is not caused by absorption in the film material, but representsoptical interference between beams reflected from the film surfaces, aphenomenon also responsible for the colors seen on soap bubbles andsometimes called a “channeled spectrum”. This phenomenon can be avoidedif desired by vacuum-sealing the encapsulating protective film tightagainst the surfaces of the lens as it is being applied.

However encapsulated, the optical component can still be held withsufficient mechanical precision through the thickness of the moisturebarrier to provide sufficient tilt and centering control in an opticalsystem. If the moisture barrier is embodied as a film to envelope theoptical component, those skilled in the art will understand that theprecision mechanical support may be designed with a plurality of groovesin the contact surfaces to allow clearance for folds and wrinkles of thefilm material proximate the optical component, but that the film may behermetically sealed against itself by pressure from a continuousmechanical contact at some convenient distance from the opticalcomponent.

A useful protective moisture barrier with adequate spectral transmissioncan also be formed by a “wet” process such as dip coating, spin coating,brushing, spraying, or electrostatically painting the optical componentwith a sufficiently thin layer of a polymer (such as PVDC) diluted in asolvent, or even with a two-component polymerizing liquid of lowviscosity.

It will be understood that two or more optical components made ofmaterials with possibly different spectral and thermal properties may beplaced together, aspheric or not, as is familiar in traditional lensdesign, to achieve any desired cancellation or enhancement ofaberrations or of spectral or thermal characteristics within the opticalsystem, and that such components may most conveniently be combinedwithin a single moisture barrier.

Optionally, the hygroscopic optical component or components can bemounted between non-hygroscopic lenses or windows in a sealed cell ifthe design permits.

The silver halides will gradually darken if exposed to visible orultraviolet light, so should be protected with a suitable spectralfilter when appropriate, possibly provided by incorporating one or moredyes or pigments directly into the moisture barrier material.

Apart from a typical lensing application, other applications for thelenses of this invention include, but are not limited to, cubic phasemasks lacking rotational symmetry for controlling wavefront shapes, andhigh NA camera objectives for use in the recently developed uncooledbolometer imaging arrays.

When a flat pellet is compressed in the conventional manner for solidmatrix sampling spectroscopy, transmission measurements may be madethrough it while it is still mounted in a metal ring forming part of thedie body, or the pellet may be pushed out of the die without concern forfractures of the compressed solid material around the edge. For lensmanufacture a superior practice will be to facilitate removal of thecompressed object from the die by providing a draft angle to therestraining die surface in which the lens is formed. To avoid problemswith the compression process, a useful embodiment (Refer to FIG. 5) usesa thin inserted molded plastic or formed metal liner 14 that can beremoved from the die body 16 along with the lens 18 that is compactedwithin it by action of the plunger 20 toward the anvil 22 of FIG. 5.Note that the removable plastic or metal liner 14 preferably has a draftangle on its outer surface that facilitates removal of the liner 14 andthe lens 18, as an assembly, from the die body 16. The lens and linertogether will be suitably coated or wrapped to protect the lens fromhumidity. A vacuum port can easily be added to the die body 16 if it isneeded for removal of air.

Another embodiment of the invention that avoids damage to the compressedlens and simplifies manufacture uses as a die body a removable molded ormachined mechanical part 24 that is itself a permanent mount for thecompacted lens, which may then be installed directly into a camera orother instrument, or into which additional lens elements 26 (typical)may be inserted to complete a compound optical system, as shown in FIGS.6 a and 6 b. As before, lens 18 is formed by compaction between plunger20 and anvil 22. A vacuum port can be added if it is needed. Again, thelens and mount together can be coated for moisture protection, or in theassembly illustrated, the mount can be held in a dry atmosphere whilethe outer lens elements are hermetically sealed into the structure.Also, the mount can be made of material transparent (e.g., acrylic orpolycarbonate) in the visible to aid in inspection.

Damage can alternatively be avoided while keeping the lens size to aminimum by constructing the die body so that it is split into two ormore sections. Those sections are clamped solidly together to form acomplete die while the powder is being compressed, but can be unclampedand separated laterally from the compacted lens, which is then simplylifted away.

Having described the invention with reference to particular embodiments,other variations will occur to those skilled in the art based on itsteachings. For example, the vacuum and sometimes the drying steps of thefabrication method may be left out in appropriate cases provided resultsstill are sensible for scattering properties. This will depend on howmuch pressure is available to squeeze down any residual air bubbles, andwhether water has any troublesome absorbing properties in the wavelengthrange of interest. Therefore, it is intended that all such variants bewithin the scope of the invention as defined by the appended claims.

1. An optical component formed of a compressible mixed powder comprisingtwo or more optical substances where the mixing ratio among said opticalsubstances has been deliberately varied from place to place within thevolume of the optical component thereby producing optically usefulvariations in the local refractive index or optical dispersion, whereinat least one of said optical substances is a metal halide.
 2. Theoptical component of claim 1 wherein one of said optical substances ofsaid compressible mixed powder is a finely ground powder compressible attemperatures lower than its melt temperature to a cohesive monolithicmass having low scattering and high transmission over predeterminedwavelengths and operating conditions.
 3. The optical component of claim2 wherein said temperature lower than the melt temperature is in therange from about 40° F. to about 520° F.
 4. The optical component ofclaim 2 selected from the group including prismatic elements, sphericallenses, aspherical lenses, cylindrical lenses, graded index lenses, lensblanks, wavefront shaping elements, aspheric add-ons, spherical andaspherical lens arrays, arrays of parallel lying cylindrical lenses,diffractive structures, diffraction gratings, holograms. Fresnel lenses,axicons, joined clusters of prisms, and combinations thereof.
 5. Theoptical component of claim 2 further including a plastic or metal linerin part surrounding the optical component and in part inserted in acompression die while the optical component is being compressed, saidliner being removable along with the optical component as a unit afterthe optical component has been formed into a monolithic mass.
 6. Theoptical component of claim 5 wherein said plastic or metal liner isfurther configured to serve as a lens mount for the optical component.7. The optical component of claim 6 wherein said lens mount is furtherconfigured for mounting at least one other optical component withrespect to said monolithic mass.
 8. The optical component of claim 2wherein said compressible finely ground mixed powder comprises potassiumbromide and germanium mixed by volume in the ratio of 70.86 to 29.14,respectively.
 9. The optical component of claim 2 wherein said finelyground mixed powder comprises an admixture of a host matrix material andat least one other material having optical properties which, when mixedwith said host matrix, provide the optical component with predeterminedoptical and physical properties.
 10. The optical component of claim 9wherein said host matrix material possesses the properties high surfaceenergy density, flowability, and deformability.
 11. The opticalcomponent of claim 10 wherein said host matrix material is selected fromthe group consisting of potassium bromide, potassium chloride, andcesium iodide.
 12. The optical component of claim 10 wherein said hostmatrix material is present in sufficient concentration to provide theoptical component with said monolithic properties and said othermaterial is present in concentrations over 2%.
 13. The optical componentof claim 11 wherein said admixture is selected from the group ofmaterials consisting of IR transmitting materials.
 14. The opticalcomponent of claim 13 wherein said IR materials is selected from thegroup consisting of alkali and metal halides and amorphous glasses. 15.The optical component of claim 2 wherein the other of said opticalsubstances comprises germanium.
 16. The optical component of claim 1further including at least part of a die body, removable along with theoptical component as a unit after the optical component has been formedinto a monolithic mass, wherein said at least part of said die body isconfigured to serve as a lens mount for at least the optical component.17. An optical component formed of compressible mixed, finely ground,powders comprising two or more optical substances where the mixing ratiohas been deliberately varied from place to place within the volume ofthe optical component thereby producing optically useful variations inthe local refractive index wherein said compressible mixed powders arecompressible at temperatures lower than their melt temperature to acohesive monolithic mass having low scattering and high transmissionover predetermined wavelengths and operating conditions and wherein atleast one of said optical substances is a metal halide, said opticalcomponent further including a plastic or metal liner surrounding theoptical component and configured to serve as a lens mount for theoptical component.